1. Technical Field
The present invention relates to a surface acoustic wave device using a quartz substrate and, in particular, to a surface acoustic wave device which can achieve excellent frequency temperature characteristic of a broadband.
2. Related Art
In recent years, a surface acoustic wave (hereinafter, referred to as SAW) device has been widely used in mobile communication terminal or other apparatuses for a vehicle. In particular, a surface acoustic wave device is required to have excellent frequency temperature characteristic of a broadband.
An SAW device, using an ST cut quartz substrate as an SAW device according to the related art, has been proposed. The ST cut quartz substrate is a cut name of a quartz substrate having a plane (XZ′ plane) which is rotated in the counter-clockwise direction from an XZ plane about a crystal X-axis by 42.75° from a crystal Z-axis, and uses an SAW (hereinafter, referred to as “ST cut quartz SAW”) that is a (P+SV) wave called a Rayleigh wave which propagates in the crystal X-axis direction. The ST cut quartz SAW device has been widely used for an SAW resonator functioning as an oscillation element, and for an IF filter disposed between an RF stage and an IC of a mobile communication terminal. FIG. 6 is a view showing the configuration of the ST cut quartz SAW resonator, in which a comb-type electrode (hereinafter, referred to as “IDT”) 102 having a plurality of electrode fingers alternately inserted therein is disposed on an ST cut quartz substrate 101, and grating reflectors 103a and 103b for reflecting an SAW are dispose at both sides of the IDT 102. The electrodes of the IDT 102 and the grating reflectors 103a and 103b each are made of Al or an alloyed metal containing Al as the main component.
In the ST cut quartz SAW device, a first-order temperature coefficient of the frequency temperature characteristic is zero, and the characteristic is indicated by a second-order curve. When the turnover temperature is set to the middle of the available temperature range, the amount of frequency variation remarkably decreases. Accordingly, it is well known that frequency stability is excellent.
However, in the above-described ST cut quartz SAW device, even though the first-order temperature coefficient is zero, a second-order temperature coefficient is relatively high −3.4(10−8/° C.2). Therefore, if the available temperature range is expanded, a problem occurs in which the amount of frequency variation sharply increases. In addition, since the electromechanical coupling coefficient k2 of the ST cut quartz SAW device is small, it is difficult to achieve characteristics of a broadband when a filter is provided.
When a piezoelectric substrate having a large electromechanical coupling coefficient k2, like that of Li2B4O7, is used in order to widen the band, the frequency temperature characteristic deteriorate, and the margin should be sufficiently ensured for security standards such as insertion loss in the operation temperature range and attenuation amount in the vicinity of a passing band, such that the manufacturing yield deteriorates.
In contrary, in Japanese Patent No. 3353742 and in Michio Kadota, “Small-sized resonator IF filter using shear horizontal wave on heavy metal film/quartz substrate”, IEEE International Frequency Control Symp. Proc., pp. 50 to 54(2002), by using an SH wave which propagates in a direction rotated in a 90° plane with respect to the propagation direction of the ST cut quartz SAW, or by using materials, having a heavier gravity than that of the piezoelectric substrate, such as W, Ta to form electrodes, an electromechanical coupling coefficient larger than that of the ST cut quartz SAW is obtained. Therefore, characteristics of a broadband can be achieved when a filter is provided.
However, in the SAW device disclosed in Japanese Patent No. 3353742 or in Michio Kadota, “Small-sized resonator IF filter using shear horizontal wave on heavy metal film/quartz substrate”, IEEE International Frequency Control Symp. Proc., pp. 50 to 54(2002), characteristics of a broadband can be achieved easier than in the ST cut quartz SAW device. On the other hand, a second-order temperature coefficient of the frequency temperature characteristic is substantially equal to that of the ST cut quartz SAW device, and the problem still remains in which the amount of frequency variation sharply increases when the available temperature range is expanded.